1. Field of the Invention
This invention relates generally to sewage wastewater transport systems. More specifically, the present invention relates to a feedback control system downstream from a pump for delivering sewage from a wet well to a force main. In the present invention, the pump and a valving system respond to the pressure or flow sensed downstream of the pump to ensure the proper operation of the pump while delivering wastewater into the force main.
2. Description of the Prior Art
Wastewater and sewage are transported to a treatment plant via a collection system. The terms "sewage" and "wastewater" as used herein are defined as the liquid waste containing both dissolved and suspended solids resulting from the discharge of toilets, baths, sinks and other fixtures in residential building or commercial establishments. Sewage also includes other suspended solid items that enter the waste collection stream, for example through open sewer drains, including trash, wood, dead animals and the like.
Sanitary sewage systems have customarily been designed to provide a gravity flow of the sewage from the point of entering the system to its final discharge. These systems consist of gravity sewer lines, pump stations and force main lines. In these systems sewage is collected in sumps or wet wells at pump stations at one or more low points in the system, from which the sewage must be pumped through force mains toward the treatment plant. Such systems are normally designed to provide velocities of at least 2 feet per second to ensure the prompt arrival of the sewage at the treatment plant or disposal site and to prevent settlement of solids in the force main.
The gravity system includes miles of pipes that generally range in size from 6 to 48 inches in diameter. Gravity sewers are constructed of relatively large diameter pipes so as to accommodate peak flows and so as to avoid being obstructed by the passage of solids contained in the sewage which are frequently stranded in the pipe system during periods of low flow, and are subsequently recaptured during later periods of high flow.
These lines feed into the main pump stations that typically are capable of pumping thousands of gallons per minute. Lift stations or pump stations are required because a point is reached where it becomes impractical to lay gravity-flow sewer lines any deeper underground. In practice, such known sewage systems typically comprise a plurality of pits for supplying each wet well and a plurality of wet wells for supplying each treatment plant via several pump stations. The pump stations discharge into force main lines. Pump and lift stations may be further provided to supplement the pressure in the force main. These force main pipes eventually lead to a treatment plant.
In prior art sewage transport systems the pumping means for delivering sewage from the wet well to the force main has taken various forms. For example, electrically or hydraulically driven submersible pumps may be located in a wet well. Typically, the type of pump used most often to deliver sewage from the wet well to the force main is a centrifugal pump specifically designed for pumping sewage and solids.
There are several kinds of centrifugal pumps including radial-flow, mixed-flow, and axial flow. In a centrifugal pump, an impeller forces the liquid being pumped into a rotary motion, and a volute (casing) directs the liquid toward the impeller. As the impeller rotates, the liquid leaves the impeller with a higher velocity and pressure than it had when it entered because the impeller produces liquid velocity and the volute forces the liquid to discharge from the pump. This velocity and pressure increase is accomplished by offsetting the impeller in the volute and by maintaining a close clearance between the impeller and the volute (at the cutwater). In this fashion, a centrifugal pump impeller slings the liquid out of the volute. The size of the impeller and the pump casing vary greatly with the type of centrifugal pump.
Every pump can add different amounts of head or pressure to the liquid it is pumping depending on the flowrate. A positive suction head exists when the liquid is taken from an open atmosphere tank where the liquid level is above the centerline of the pump suction, commonly known as a flooded suction. A suction lift exists when the liquid is taken from an open atmosphere tank where the liquid level is below the centerline of the pump suction. Pump Performance Curves are produced by a pump manufacturers to show the relationship between flow and total dynamic head, the efficiency, the Net Positive Suction Head Required (NPSHR), and the Brake Horse Power Required (BHPR). A pump curve, as in FIG. 1, is the characteristic curve showing the ability of the pump to add head at different flow rates. As shown by the performance curve, a higher head corresponds to lower flow and a lower head corresponds to higher flow. Furthermore, a lower flow corresponds to lower horsepower requirement and a higher flow corresponds to higher horsepower requirement. A system curve represents the head losses due to losses and friction as well as the static lift in the pumping situation of interest and may be overlaid on the pump curve. The point where the curves cross is the operating point of the pump in the particular system.
Because a centrifugal pump is a variable displacement pump, the actual flow rate achieved is directly dependent on the total dynamic head against which the pump must work. The flow capacity of a centrifugal pump also depends on the pump design, the impeller diameter and the pump speed and the net positive suction head. The Net Positive Suction Head Required (NPSHR) is a function of the pump design at the operating point on the pump performance curve. The Net Positive Suction Head Available (NPSHA) is a function of the pump suction piping system and the amount of lift the pump is performing. However, if the NPSHA is less than the NPSHR, the pump will cavitate.
Cavitation is a major problem that may occur in wastewater pumping systems. Cavitation may occur in two different forms: suction cavitation and discharge cavitation. Suction cavitation occurs when the pump suction is under a low pressure/high vacuum condition where the liquid turns into a vapor at the eye of the pump impeller. This vapor is carried over to the discharge side of the pump where it no longer sees vacuum and is compressed back into a liquid by the discharge pressure. This imploding action occurs violently and attacks the face of the impeller. An impeller that has been operating under a suction cavitation condition has large chunks of material removed from its face causing premature failure of the pump.
Discharge cavitation occurs when the pump discharge is extremely high. It normally occurs in a pump that is running at less than 10% of its best efficiency point. The high discharge pressure causes the majority of the fluid to circulate inside the pump, instead of being allowed to flow out the discharge. As the liquid flows around the impeller it must pass through the small clearance between the impeller and the pump cutwater at extremely high velocity. This velocity causes a vacuum to develop at the cutwater and turns the liquid into a vapor. A pump that has been operating under these conditions shows premature wear of the impeller vane tips and the pump cutwater. In addition, due to the high-pressure condition, premature failure of the pump mechanical seal and bearings may occur and under extreme conditions will break the impeller shaft.
Another problem peculiar to wastewater collection systems is blockage of the system and components by the solids within the sewage. Based on the determination that impellers suited for the conveyance of solid additions tend to be blocked under unfavorable circumstances, the prior art has contemplated solutions to the problem. U.S. Pat. No. 5,348,444 to Metzinger discloses a single-blade impeller for centrifugal pumps which fulfills the basic prerequisite of freedom from blockages, and has a calibrated design which attempts to reduce the cavitation present in the intake zone of the single-blade impeller.
Another problem peculiar to wastewater collection systems is the unpredictable flow rate of wastes through the system. Currently used collection and treatment processes of necessity must operate with flow rates that vary from as much as 250% increase over average daily flow during peak periods to as low as 10% of average daily flow during night and early morning periods. It is therefore required that a pump station centrifugal pump be rated to handle peak flow capacity to pump wastewater from the wet well to the force main under peak flow conditions. The same pump however, must also be able to effectively pump wastewater to the force main under low flow conditions.
Thus, the conveying conditions of such a centrifugal pump normally undergo continuous change. The positive pressure head varies between a maximum and a minimum value and the working point of the centrifugal pump, the intersection of the constant pump characteristic line and the variable system characteristic line, accordingly is likewise subjected to the change. In most applications, it is preferred that centrifugal pumps are operated at constant rpm. However, in order to address the need to overcome changing flow conditions, the prior art has contemplated using multiple pumps or a variable speed pump. Both of these options increase the cost for components and maintenance and operation of a pump station.
Another problem peculiar to wastewater collection systems is the backpressure encountered from force mains. It is a requirement in waste collection systems, that under peak flow or low flow conditions, the pump must discharge the wastes at a sufficient pressure to overcome the pressure of the force main, typically 0-100 psig.
Premature failure of the centrifugal pump due to damage from blockage or cavitation is undesirable. Failure of the pump to operate, or failure to fulfill flow requirements can cause sewage back-up and overflow from the wet well with obvious undesirable consequences to the environment in the vicinity of the pump station.
One prior art solution to overcome pump failure is to provide a standby engine-driven generator at the pump station to provide emergency electric power in case of a power outage on the utility lines. However, since the electric motors to be powered by the engine-driven generator typically can require for startup up to 400 percent of the power needed for normal running, the engine-driven generator must be an oversized expensive machine which can cost a considerable amount of money.
Such equipment is infrequently used and, even then, seldom at full capacity. Another prior art solution is described in U.S. Pat. No. 4,529,359 to Sloan, which provides sewage pumping means for a lift station that provides standby hydraulic pumps to supplement existing electrical or hydraulic submersible pumps. This equipment can also be quite expensive.
These solutions also do not address the basic problems of clearance of blockages and cavitation that most often cause the premature failure of the pump.